The noise signal forces the laser to operate in a laser mode within the noise band.

In this device, a suppression unit suppresses a noise component from the audio power spectrum by using the detection result of the audio-existing band and the noise band in the audio power spectrum including the noise component.

The noise band C2 is subtracted from the high frequency band C1 to produce a signal band D that contains the signal.

Any signals outside the noise band are removed from the transmission signal by a narrow-band filter (8).

The filtered image B2 is subtracted from the original image A to produce a noise band C2 that contains noise with very little signal.

The power spectral density and power per unit bandwidth is constant over the noise bandwidth.